Chem 3A-Burdge Worksheet- Chapter 17-Redox Reactions Page 6 of 7 barcess aqua re

Question

Chem 3A-Burdge Worksheet- Chapter 17-Redox Reactions Page 6 of 7 barcess aqua regia can be boiled off leaving solid HAuCl (chloroaurie acid). Chloroaurie acid can then be dissolved in water and reacted with sulfur dioxide to form solid gold precipitate. The unbalanced chemical equation for the precipitation of solid Au is given below: AuC4-(aq) + SO2 (g)- Au(s) + SO42-(aq) + Cl-(aq) For the following questions, involving the balancing of this reaction, balance in acidic conditions (a) (6 pts) Write the balanced reduction half-reaction for the reaction above b) (6 pts) Write the balanced oxidation half-reaction for the reaction above (c) (6 pts) Write the balanced overall equation, with phase symbols, for the reaction above: (d) (10 pts) If 331.20 g of HAuCl4 (339.78 gmol-1) was reacted with excess sO2 in exccess HO and 186.3 g of pure Au was obtained from the reaction above, what is the percent yield of gold? Hint: This part does not require the above reaction to be balanced correctly because 1 mol of HAsC contains I mol Au

Explanation / Answer

a)

reduction --> gain i nelectrons

AuCl4- = Au + Cl-

balance

AuCl4- = Au + 4Cl-

balance e-

3e- + AuCl4- = Au + 4Cl-

b)

oxidation is loss of e-

SO2 = SO4-2

balance O adding H2O

2H2O + SO2 = SO4-2

balance H by H+

2H2O + SO2 = SO4-2 + 4H+

balance charge

2H2O + SO2 = SO4-2 + 4H+ + 2e-

c)

overall equation

3e- + AuCl4- = Au + 4Cl-

2H2O + SO2 = SO4-2 + 4H+ + 2e-

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6e- + 2AuCl4- = 2Au + 8Cl-

6H2O + 3SO2 = 3SO4-2 + 12H+ + 6e-

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6e- + 2AuCl4- + 6H2O + 3SO2 = 3SO4-2 + 12H+ + 6e- + 2Au + 8Cl-

cancel common terms

2AuCl4- + 6H2O + 3SO2 = 3SO4-2 + 12H+ + 2Au + 8Cl-

d)

mol of HAuCl4 = mass/MW = 331.2/339.78 = 0.9747

mol of AuCl4- = 0.9747

ratio is 2 mol of AuCl4 = 2 mol of au

0.9747 mol --> 0.9747 mol of au

mass = mol*MW = 0.9747*196.96655= 191.98g of Au

% yield = real/theoretical * 100% = 186.3/191.98*100 = 97.041 %

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